No short-term effect of sinking microplastics on heterotrophy or sediment clearing in the tropical coral Stylophora pistillata

Investigations of encounters between corals and microplastics have, to date, used particle concentrations that are several orders of magnitude above environmentally relevant levels. Here we investigate whether concentrations closer to values reported in tropical coral reefs affect sediment shedding and heterotrophy in reef-building corals. We show that single-pulse microplastic deposition elicits significantly more coral polyp retraction than comparable amounts of calcareous sediments. When deposited separately from sediments, microplastics remain longer on corals than sediments, through stronger adhesion and longer periods of examination by the coral polyps. Contamination of sediments with microplastics does not retard corals’ sediment clearing rates. Rather, sediments speed-up microplastic shedding, possibly affecting its electrostatic behaviour. Heterotrophy rates are three times higher than microplastic ingestion rates when corals encounter microzooplankton (Artemia salina cysts) and microplastics separately. Exposed to cysts-microplastic combinations, corals feed preferentially on cysts regardless of microplastic concentration. Chronic-exposure experiments should test whether our conclusions hold true under environmental conditions typical of inshore marginal coral reefs.


Table S1
Microplastic concentrations reported in the environment on coral reef areas since 2015 up to February 2021. Table S2 Mean (±SE) single-pulse concentrations of microplastics, sediments, and Artemia salina cysts reached per treatment at the start of the experiments. Table S3 Microplastic concentrations used in previously published experiments exposing Corals to microplastics, compared to those used here.

Figure S1
Photographs of the a) experimental setup of the feeding experiments, b) corals' 3D models, c) calcareous sediment particles, d) stained PET microplastics, e) decapsulated A. salina cysts, g) Microplastic particles trapped in mucus strains and in h) gas bubbles.

Figure S2
Output of Bayesian Poisson hurdle model for numbers of polyps touched by microplastics and sediments when these are added separately to corals.

Figure S3
Output of Bayesian Poisson hurdle model for numbers of polyps touched by sediments when these are added separately to corals or in 50:50 mixtures with microplastics. Figure S4 Output of Bayesian Poisson hurdle model for numbers of polyps touched by microplastic particles when these are added separately to corals or in 50:50 mixtures with sediments.

Table S4
Output of models comparing i) heterotrophy rates in corals exposed to A. salina alone at 100 items L -1 and corals exposed to even mixtures of A. salina and microplastics at 50 items L -1 each and ii) microplastic ingestion rates in corals exposed to only microplastics at 100 items L -1 and corals exposed to even mixtures of A. salina and microplastics at 50 items L -1 each.

Table S5
Mean values (± SE) of selected physicochemical parameters of the seawater in the coral maintenance tank for the duration of the experiment 01.08. -31.12.2018

Figure S5
Mean number of particles (± SE) visible to the cameras at the beginning of the experiments (hour = 0) exposing S. pistillata fragments to reef sediments (treatment A), microplastics (treatment B), and 50:50 mixtures of reef sediments and microplastics (treatment C). A Poisson GLM detected no significant differences among treatments (p = 0.39).

Table S6
Outputs of the linear model testing whether the number of particles cm -2 coral differed significantly among treatments A -C.

Figure S6
Proportion of retracted polyps recorded hourly for 12 hours in a parallel experiment run on Pocillopora damicornis exposed to a) stained and b) unstained irregular PET microplastics. No significant differences were observed among hours or between the type of particles (Table S7).

Table S7
Statistical outputs of a binomial generalised linear model (GLMER) fitted to data obtained during a parallel experiment exposing P. damicornis to stained (n = 9 fragments) and unstained (n = 5 fragments) irregular PET microplastics. The model tests whether the proportion of retracted polyps within colonies changed hourly for 12 hours, and whether this change was contingent on microplastic staining.

Figure S7
Model validation plots for the GAMM with binomial distribution fitted to test whether the proportion of polyps retracted over time differed when exposed to control experimental conditions, sediments, and microplastics.
Hall This study Treatment B -2.86 (± 0.41) particles L -1 Treatment C -1.68 (± 0.16) particles L -1 Treatment D -0.00 (± 0.00) particles L -1 Treatment E -50.00 (± 0.00) particles L -1 Treatment F -75.00 (± 0.00) particles L -1 Treatment G -100.0 (± 0.00) particles L -1 Figure S1 Fig. S1. a) Experimental setup of the feeding experiments including the 1L Weck jars within which fragments were suspended. b) Example of the fragments' 3D-model generated in Autodesk ReCap Pro in order to compute colony surface area. c) Reef sediment particles, d) stained PET microplastics used in the experiments exposing corals to sediments and/or microplastics, e) decapsulated A. salina cysts, f) stained PET microplastics used in the experiments exposing corals to A. salina and/or microplastics, all photographed through a stereomicroscope. g) Microplastic particles trapped in mucus strains and in h) gas bubbles. Gas bubbles were not exclusively (yet more often) produced by corals exposed to microplastics, but also occasionally produced by control corals likely in response to the lighting used in the experimental set up.  A and B), sediments and microplastics differ in the number of coral polyps touched per particle. Treatment A-Sediments is nested in the intercept, and b_total.area refers to the corals' surface area.

Figure S3
Outputs

Figure S4
Outputs   Table S4   Table S4. Output of models comparing i) heterotrophy rates in corals exposed to A. salina alone at 100 items L -1 and corals exposed to even mixtures of A. salina and microplastics at 50 items L -1 each, and ii) microplastic ingestion rates in corals exposed to only microplastics at 100 items L -1 and corals exposed to even mixtures of A. salina and microplastics at 50 items L -1 each.  2.63 ± 0.15

Figure S5
Although we the same number of particles (n = 35) was supplied to all fragments in the sediments and/or microplastics experiments, the number of particles that actually settled on the corals and were visible to the camera at the onset of the experiment (hour 0) differed across fragments. To test for potential biases this may have caused in our results, we tested whether the number of settled particles in view differed among treatments using a Poisson generalised linear model (GLM) suitable for discrete responses. The dispersion parameter of the GLM (i.e. 2.1 indicated no overdispersion, and the output revealed no significant differences among treatments (p = 0.39, Fig  S2).   . Proportion of retracted polyps recorded hourly for 12 hours in a parallel experiment run on Pocillopora damicornis exposed to a) stained and b) unstained irregular PET microplastics. No significant differences were observed among hours or between the type of particles (Table  S7). Table S7   Table S7. Statistical outputs of a binomial generalised linear model (GLMER) fitted to data obtained during a parallel experiment exposing P. damicornis to stained (n = 9 fragments) and unstained (n = 5 fragments) irregular PET microplastics. The model tests whether the proportion of retracted polyps within colonies changed hourly for 12 hours, and whether this change was contingent on microplastic staining.

Figure S7
Fig. S7. Model validation plots for the GAMM with binomial distribution fitted to test whether the proportion of polyps retracted over time differed when exposed to control experimental conditions, sediments, and microplastics.